Respirators are commonly worn over the breathing passages of a person for at least one of two common purposes: (1) to prevent impurities or contaminants from entering the wearer's breathing track; and (2) to protect other persons or things from being exposed to pathogens and other contaminants exhaled by the wearer. In the first situation, the respirator is worn in an environment where the air contains particles that are harmful to the wearer, for example, in an auto body shop. In the second situation, the respirator is worn in an environment where there is risk of contamination to other persons or things, for example, in an operating room or clean room.
A variety of respirators have been designed to meet either (or both) of these purposes. Some of these respirators have been categorized as being “filtering face-pieces” because the mask body itself functions as the filtering mechanism. Unlike respirators that use rubber or elastomeric mask bodies in conjunction with attachable filter cartridges (see, e.g., U.S. Pat. RE39,493 to Yuschak et al.) or insert-molded filter elements (see, e.g., U.S. Pat. No. 4,790,306 to Braun), filtering face-piece respirators have the filter media cover much of the whole mask body so that there is no need for installing or replacing a filter cartridge. Conventional filtering face piece respirators have regularly comprised non-woven webs of thermally-bonding fibers to furnish the mask body with its intended cup-shaped configuration. These filtering face-piece respirators are relatively light in weight and easy to use. Examples of patents that disclose such filtering face-piece respirators include U.S. Pat. No. 7,131,442 to Kronzer et al, U.S. Pat. Nos. 6,923,182 and 6,041,782 to Angadjivand et al. U.S. Pat. Nos. 6,568,392 and 6,484,722 to Bostock et al., U.S. Pat. No. 6,394,090 to Chen, U.S. Pat. No. 4,807,619 to Dyrud et al., and U.S. Pat. No. 4,536,440 to Berg.
Conventional filtering face-piece respirators also have used an open-work plastic mesh structure to provide mask shape and support for the filter media. Examples of patents that describe plastic mesh-type support structures include U.S. Pat. No. 4,873,972 to Magidson et al., U.S. Pat. No. 4,850,347 to Skov, and Des. 285,374 to Huber et al. Commercially available respirators that use open-work plastic meshes as the support structure include, for example, the 2200-2500 Series and the EX-ON Series particulate respirators sold by Moldex-Metric Inc. of Culver City, Calif. The known mesh support structures for filtering face-piece respirators all exhibit positive Poisson ratios—that is, they contract laterally when stretched and expand laterally when compressed. These lateral contractions and expansions may be noticed in a finished product that has been subject to heat and pressure during mask body manufacture.